Added README

README.md

+lbmpy-walberla
+==============
+[![pipeline status](https://i10git.cs.fau.de/pycodegen/lbmpy_walberla/badges/master/pipeline.svg)](https://i10git.cs.fau.de/pycodegen/lbmpy_walberla/commits/master)
+
+Formulate lattice Boltzmann methods in lbmpy and generate optimized C++ Code for waLBerla from it.
+Look at the waLBerla documentation on how to use it.
+
+Install with
+```bash
+pip install lbmpy-walberla
+```
+and enable WALBERLA_BUILD_WITH_CODEGEN in waLBerla's CMake.